Add "preload" feature

[pinx]

We have to translate Query.preload and Repo.preload into something like AQL subqueries

from p in Post,
  join: c in assoc(p, :comments),
  preload: [comments: c]

should translate into

FOR p IN posts
  FOR c IN comments
  FILTER p._key == c.post_id
RETURN { post : p, comments : c }

[mpoeter]

IMHO this should work already.

[pinx]

I added it to #8 as an undocumented feature.